[fix] expose Vite.js mode from $app/env

.changeset/silly-jokes-hug.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+Expose Vite.js mode from \$app/env

documentation/docs/05-modules.md

@@ -7,12 +7,13 @@ SvelteKit makes a number of modules available to your application.
 ### $app/env
 
 ```js
-import { amp, browser, dev, prerendering } from '$app/env';
+import { amp, browser, dev, mode, prerendering } from '$app/env';
 ```
 
 - `amp` is `true` or `false` depending on the corresponding value in your [project configuration](#configuration)
 - `browser` is `true` or `false` depending on whether the app is running in the browser or on the server
 - `dev` is `true` in development mode, `false` in production
+- `mode` is the [Vite mode](https://vitejs.dev/guide/env-and-mode.html#modes), which is `development` in dev mode or `production` during build unless configured otherwise in `config.kit.vite.mode`.
 - `prerendering` is `true` when [prerendering](#ssr-and-javascript-prerender), `false` otherwise
 
 ### $app/navigation

packages/kit/src/runtime/app/env.js

@@ -6,6 +6,10 @@ export const browser = !import.meta.env.SSR;
  * @type {import('$app/env').dev}
  */
 export const dev = !!import.meta.env.DEV;
+/**
+ * @type {import('$app/env').mode}
+ */
+export const mode = import.meta.env.MODE;
 /**
  * @type {import('$app/env').amp}
  */

packages/kit/types/ambient-modules.d.ts

@@ -15,6 +15,12 @@ declare module '$app/env' {
 	 * `true` when prerendering, `false` otherwise.
 	 */
 	export const prerendering: boolean;
+	/**
+	 * The Vite.js mode the app is running in. Configure in `config.kit.vite.mode`.
+	 * Vite.js loads the dotenv file associated with the provided mode, `.env.[mode]` or `.env.[mode].local`.
+	 * By default, `svelte-kit dev` runs with `mode=development` and `svelte-kit build` runs with `mode=production`.
+	 */
+	export const mode: string;
 }
 
 declare module '$app/navigation' {